Cooperative surveillance systems notably allow determination of the position of a plurality of aircraft by relying on satellite positioning systems such as the Galileo, GPS or GLONASS system. These systems can operate in broadcast mode or in connected mode. An example of a system operating in broadcast mode is the ADS-B system, ADS-B standing for “Automatic Dependant Surveillance-Broadcast”. In this type of system, the aircraft are equipped with transponders. They assess their positions and transmit them regularly with other information to other aircraft or to stations on the ground. If some aircraft are not equipped with transponders, the TIS-B service, TIS-B standing for “Traffic Informations Services-Broadcast”, allows information notably detected by means of ground radar to be sent to the various aircraft.
Cooperative surveillance systems can be used notably for implementing equipment allowing collisions between aircraft to be avoided and are usually denoted by the acronym TCAS, standing for “Traffic Collision Avoidance System”. When two aircraft are close to one another, the TCAS informs the pilot of the first aircraft about the proximity of a second aircraft. A manoeuvre called spacing can then be implemented. These means generally belong to the category of safety nets, which are intended to increase air safety by providing a surveillance function and emergency manoeuvres, which are totally independent of the navigation.
Another use for cooperative surveillance means is beginning to appear, this time integrated in the navigation functions, in order to improve air navigation, facilitate traffic, and maintain or increase air safety within a context of constantly increasing traffic. The management of the spacing and separation in relation to the surrounding traffic is thus delegated to the crew of the aircraft by air traffic control. The relevant capabilities are integrated in the onboard navigation means, which integrate the spacing and separation constraints in their manoeuvre capabilities.
Several types of spacing manoeuvres can thus be considered. They are usually based on a speed calculation by a piece of Traffic Management equipment, which may or may not be coupled to the TCAS traffic surveillance equipment, and rely on position and ground speed measurements and on indications of time and trajectory length which are provided by the FMS navigation system, FMS standing for “Flight Management System”. A lateral trajectory, that is to say a trajectory aimed at modifying the route distance between two points on the flight plan, can also be determined by the FMS, and is taken into account in the speed calculation essentially by a distance calculation along said trajectory.
When a spacing manoeuvre is required by air traffic control and needs to be accepted by the crew on the aircraft, it is advisable to determine the feasibility thereof, that is to say to determine whether it can be implemented in practice. This feasibility assessment and the robustness of execution thereof come up against several limitations. Firstly, the performance of the aircraft is known at best only from the theoretical flight envelope in terms of minimum and maximum speeds. Next, the impact of the wind on speed holding is not taken into account, or is taken into account only imperfectly, because to do so well requires measurement not only of the wind but also of the projection thereof on the lateral trajectory. Finally, speed holding is strongly coupled to the vertical profile, that is to say to the change in altitude along the trajectory, with the vertical profile limiting the acceleration and deceleration capabilities and also the minimum flyable speed along this profile. For this reason, the feasibility assessment for a spacing manoeuvre at speed or for a lateral trajectory is necessarily simplified and does not take account of the various factors which can disrupt the spacing manoeuvre. Moreover, execution of the manoeuvre is based on a speed calculation which does not take account of these various disruptions and which can thus lead to failure of the manoeuvre. It is therefore necessary to anticipate these factors and then to monitor feasibility during execution.